This invention relates to a photoreceptor and, particularly, to an electrophotographic photoreceptor.
Further, this invention relates to an image forming method and the apparatus therefor and, particularly, to an image forming method suitable for forming a dot-image by dotwise exposing of a beam for exposure according to a digital signal and the apparatus therefor.
Heretofore, as the photoreceptors applicable to electrophotography, those having the so-called low .gamma. type light decaying characteristics, that is slow in light decay, are known, as shown in FIG. 6A and, on the other hand, those having the so-called high .gamma. type light decaying characteristics, that is slow in light decay at the initial stage of an image exposure and sharp in the light decay at the middle and late stages are also known, as shown in FIG. 6B.
The reason why the high .gamma. type photoreceptors can show the above-mentioned excellent characteristics is still not fully made clear. It is, however, presumed that, at the initial stage of the image exposure, carriers produced on the surface of a photosensitive substance which particularly, made of a photoconductive organic pigment, are trapped thereon for a little while to delay the light decay and the carrier trapping is then saturated at the middle and late stages of the exposure to produce resultingly an avalanche phenomenon at a stroke, so that the light decaying characteristics may be shown to decay almost straight.
The characteristic of the above-mentioned high .gamma. type photoreceptor is to have a maximum value of the differential coefficient on the light decay curve drawn by plotting the surface potentials of the photoreceptor to the exposure amount; there, the both axes of the surface potential and the exposure are not ploted in linear scales not in terms of logarithm. The term, `a light decay curve`, herein defines the relation between the surface potential of photoreceptor and an exposure amount when the photoreceptor is given a certain light exposure and, in the curve, the surface potential of photoreceptor is taken on the ordinate and an exposure irradiated to the surface of the photoreceptor is taken on the abscissa. In FIG. 6B, the exposure is taken in terms of exposure time. With respect to the curve, the term, `differential coefficient` means an inclination of the tangential line to the light decay curve (a) and is defined approximately as a absoute value of .DELTA.V/.DELTA.I where the surface potential of a photoreceptor is changed from V.sub.O into V.sub.0 +.DELTA.V when a certain exposure I.sub.O is increased by .DELTA.I. FIG. 7 shows the differential coefficient-exposure characteristics on a light decay curve obtained in the above-described method of obtaining an approximate differential coefficient, wherein curve A having the maximum value corresponds to curve (a) shown in FIG. 6B and curve (A') corresponds to light decay curve (a') shown in FIG. 6A.
In the engineering field of electrography in recent years, the image forming methods have been extensively researched and developed by adopting the digital system capable of readily making the improvements of image qualities and the conversions and compilations of images so that high-quality images can be formed. In such image forming methods, the photoreceptors having the above-mentioned high .gamma. type light decaying characteristics can advantageously be used. For example, the light beam, of a laser, preferably, a semiconductor laser, a LED array and a liquid crystal shutter are modulated by the digital image signals given through a computer from an original document subject to copy and a uniformly charged photoreceptor is exposed dot-wise by the modulated light image, so that a dot-shaped electrostatic latent image may be formed. When a dot-shaped image is formed by processing the resulting latent image with toner, preferably, in a reversal development, an exposure is usually made dotwise at a luminance within the range of 1 to 5 mw with an extremely narrow dot width within the range of 50 to 100 .mu.m.
In the case of such a narrow dot width exposure as mentioned above, the high .gamma. type photoreceptors have a short and sharp skirt portion in both of the potential distribution of the dot-shaped electrostatic latent image and the density distribution of the dot-shaped image. Therefore, the photoreceptors of this type are advantageous to the digital system image formation.
The image forming apparatuses applicable with the high .gamma. type photoreceptors include, for example, the apparatus schematically shown in FIG. 9. In the figure, referential numeral 1 is a photoreceptive drum, 2 is a charger, 3 is a light input signal, 5 is a developing unit, 6 is a cleaning section, 7 is a transfer paper, and 8 is a transfer charger. The image forming apparatus shown in FIG. 10 is that disclosed in Japanese Patent Publication Open to Public Inspection-hereinafter referred to as Japanese Patent O.P.I. Publication-No. 172863/1989, wherein to the light input signal 3 of the apparatus shown in FIG. 9, another light input 4 having a uniform intensity for sensitization use is added.
In the meantime, the above-described high .gamma. type photoreceptors are advantageous to the digital recording systems. However, these photoreceptors have the following problem. As shown by (b) in FIG. 6B, there is an induction period from starting an exposure to the time of rapidly lowering the surface potential, i.e. the time of rapidly causing an avalanche phenomenon, so that an absolute sensitivity becomes low. Therefore, the exposure should be increased more than usual.
As shown in FIG. 10, therefore, when light input 4 for sensitization is applied, it is reasonably expectable to obtain the effects on the compensation for the short exposure and the increase in apparent sensitivity. However, as the original light input 3 and another light input 4 are applied at the same time, an interference is occured between the two inputs, thereby producing an unevenness on the electrostatic latent image, or the image, so that a uniform image cannot be formed.
It is an object of the invention to provide a method capable of improving a sensitivity, forming a uniform latent image and obtaining an excellent image; and to provide the apparatus therefor.
The photoreceptors showing the above-described high .gamma. type light decaying characteristics are excellent in the reproducibility of thin-line portions or character portions among the image areas as compared to the other normal type photoreceptors. However, the photoreceptors of this type have the defect that the density of a over-all solid image is lowered.
Another object of the invention is to provide a method of obtaining an image excellent in the resolving power of the thin-line portions and satisfactory in the density of an overall solid image, not losing the special features of the photoreceptors having the above-described high .gamma. type light decaying characteristics.
When the photoreceptor having the characteristics shown by curve A exhibited in FIG. 7 is exposed to laser beam, the surface potential V.sub.H thereof in an unexposed area is still high in the initial stage. However, the potential V.sub.H is lowered as the photoreceptor is repeatedly used. On the other hand, the surface potential V.sub.L thereof in an exposed area is also lowered accordingly. In other words, the photoreceptors having the high .gamma. type light decaying characteristics are not effectively used, because they have the defect that the light decay curve is varied to deteriorate the receptors in the course of repetition use, although they have the special features such as a sharp light decay in the late stage of exposure and the high gamma characteristics. For example, they have the following defects. A fog is produced on an image area and an erratic developability is also produced, because V.sub.H is seriously lowered in the order of 100 copy cycles and V.sub.L is high in the initial stage.
In addition to the above, in electrophotographic processes, ozone or other active substances are derived from the charging by a corona discharge and the photoreceptors are affected by the above-mentioned active substances so as to raise the problems such as the deterioration of charging characteristics and sensitivity, and the raise of residual potentials. Particularly in the repetition use, the time of exposing the photoreceptors to ozone or other active substances are increased progressively and, therefore, the lowering of the charging characteristics and sensitivity, and the residual potential's rise occur seriously.
Even if adding the antioxidant as described in, for example, Japanese Patent O.P.I. Publication Nos. 130759/1981, 73744/1982 and 122444/1982 for the countermeasure to solve the above-mentioned defects, there still remain the following defects. The photoreceptors having the excellent charging characteristics will cause the deterioration of the initial sensitivity; those having an excellent initial sensitivity will not improve the deterioration in repetition use; and those having the less deterioration in repetition use are not satisfactory in the initial sensitivity and the charging characteristics.
A further object of the invention is to provide a photoreceptors having the above-described high .gamma. type light decaying characteristics, which are capable of stabilizing the potentials in repetition use and, at the same time, improving the initial sensitivity.
Notwithstanding the conventional high .gamma. type photoreceptors are required to have still more higher .gamma. type light decaying characteristics, none of any photoreceptors capable of satisfying the requirement is still practically known. In addition to the above, though the stability in repetition use is also demanded, it is the actual situation where any satisfactory countermeasure is still not taken.
Yet another object of the invention is to provide a photoreceptor capable of improving the sensitivity to make the high .gamma. type light decaying characteristics more higher and stabilizing the characteristics in repetition use.